Free-machining stainless steels



Sept. l0, 19.68

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ARTHUR MOS/(DW/TZ, CURTIS W. KOVALHand RALPH 6. WELLS By kin-vv Attorney Sept. l0, 1968 A. MosKoWlTz ETAL 3,401,035

FRr-.MACHNNG STAINLESS STEELS 17 Sheets-Sheeft .'5

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FREE'MACHINING STAINLESS STEELS Filed Dec.

0 Smal/ Hanf:

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Sept. 10, 1968 Filed Dec.

Chromium Equiva/en t A. MOSKOWITZ ET AL FREEMACHINING STAINLESS STEELS 1;? Sheets-Sheet /o /5 2o 25 so as 4o Fe rr/'te Content, Volume Percent F I6. 7T

/NvE/vrons ARTHURHS/(QVITZ, CURTIS #KOI/ACH AND RALPH 6. WELLS Wp /vx/ Attorney A. MosKown'z ETAL 3,401,035'-A FREE-MACHINING STAINLESS STEELS Sept. l0, 1968.

Filed Dec.

17 Sheets-Shag?. 6

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un., Mwgf Fer/'ile Conenf, Volume Percenl INVENTONS ARTHUR NSKOWITZ WHY/S Y KOV6H AND RLPH G WELLS Attorney Sept 10, 1968 A. MosKowlTz ETAL FREE-MACHINING STAINLESS STEELS 17 Sheets-Sheet '7 Filed Dec. 7, 1967 Sulfur, Weight Percent FIG. .9.l

IN VEN 7' 0K8 RHUR a 5 7;. l. 6. l a O a4 l l a l R O 7 S l /a n 0 M A 2- 2 l l... 0 0 0 0 0 0 0 p. n m 9 a 1.0 1.2 1.6 1.6 2.0 2.2 2.4 2.6 2.a 50 Man anese Wei M r nl g 9 P' ce cular/s ar.

RALPHG. WELLS WSKOWITZ, KOYCH AN@ FIG. l0.

17 Sheets-Sheet 8 Filed Dec. 7, 1967 IVE/V70 RTM/R NOSKOMTZ WHT/S IKKOYGH AND RALPH 6. 'EL L5 .t GF4.

Sept. 10, 1968 A. MosKowlTz ETAL FREE-MACHINING STAINLESS STEELS` l 17 Sheets-Sheet 9 Filed Dec'. 7, 1967' Muff-wraps` .ann/un Mos/rom rz, cu/rr/s w. Kol/,40H and mand WJ!" 'tv/1.4 717s RALPH G. WELLS y J 7Affarney FREE-MACHIN ING STAINLESS STEELS 17 Smeets-sheet` 1o Filed Dec.

mmmms v FIG. I4.

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/NVENTGRS ARTHUR NOSKW/TZ CURTIS W. KOVCH AND RALPH 6. WELLS Grades mi2@ *Y ud l Grades Genel ral Purpose Free -machin/'ng Fara/'ng Grades -Bi This Invention r A f tornay Sept. l0, 1968 A. MosKowlTz ETAL 3,401,035

FREE-MACHINING STAINLESS STEELS Filed Deo. 7, 1967 17 Sheets-Sheet 11 l 9 H3 .Tv M

Hear /84 Ain/S l 0 Hear H89 Ain/5*! INI/enfans Aarau wslrolwrz CURTIS l' KUVACN AND RALPH E. WELLS a A f fargey sept. 1o, 196s A. MOSKOWTZ ETAL FREE-MACHINING STAINLESS STEELS 17 Sheets-Sheet lz Filed Dec. '7, 1967'4 4 3 2 UW ...my 5 n cs mwmr, Mmmm u/ l 2 mnmw u/ uw! .f rs nu AMM ck E n.. .w 6. l l F A f fanny Weight Percent Mela! in Su/fide Sept. 10, 1968 A; McsKowlTz ETAL l I 3,401,035

FREE-MACHINING STAINLESS STEELS Filed Dec. 7. 1967 17 Sheets-Sheet 13 El Manganese Weight Percent Mela! in Su/fde l Chrom/'um la A i A ...A-1f A Iran A4 l l l l I s 4 5 FIG. /6 Mn/S Raf/a Manganese Chromium l I 0 e 7` e F IG. I9. Mn/.s Raffa INVENTMS AR THUN HOSKOW/TZ CURTIS lY-KOVCH AM RALPH GJYE A t farnay Esquel suueumb 17 Sheets-Sheet. 14

A. MOSKOWITZ ETAL FREE-MACHINING STAINLESS STEELS x-M-x [wand any Jo uoyanpoy Sept. l0, 1968 Filed Dc.

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Sept' 10, 1968 A. MosKowlTz ETAI. 3,401,035

FREE-MACHINING STAINLESS STEELS Filed Dec. 7, 1967 17 Sheets-Sheet 15 x-'x-x Juaalad any lo aoyonpay o 'n o u, 9, I v v a g l I I l l I vv-v spunod-Jao; 'wduaug nodal] VanN-,1 {dau/,7 u Q n 5v N l S n I l I l I l n--n-u wund no uoduo/g Q v E; 3 w Q S h o I l l I l l lq' '4 u l N v v v y .fsa p ay s s g a a l l l I I l l I u I 'a I 0. E l QE l N u y i I l l -2 c :a o. v: o

l l I l I l Q o e o o o Q n 2 z s Sept. l0, 1968 A. MosKowlTz ETAL 3,401,035

FREE'MACHINNG STAINLESS STEELS Filed Dec. 7, 1967 17 Sheets-Sheet 16 H x uaoJad any Jo uoyanpay CURT/S f KOVCH AND RALPH G. WELLS By Y Arney sept. 1o, 1968 Filed Dec. '7, 1967 Weight Loss, Grams per Square/loch A. MosKowlTz ETAL 3,401,035

FREE-MACHINING STAINLESS STEELS 17 Sheets-Sheet 17 anruulgvogrfwr cwrr/s mxovAcH Alva RALPH s. WEL/.s

United States Patent Oce 3,401,035 Patented Sept. 10, 1968 3,401,035 FREE-MACHINING STAINLESS STEELS Arthur Moskowitz, Curtis Walter Kovach, and Ralph Gordon Wells, Pittsburgh, Pa., assignors to Crucible Steel Company of America, Pittsburgh, Pa., a corporation of New Jersey Continuation-impart of applications Ser. No. 472,756,

July 6, 1965, and Ser. No. 556,056, June 8, 1966.

This application Dec. 7, 1967, Ser. No. 693,044

7 Claims. (Cl. 75-126) ABSTRACT F THE DISCLOSURE This invention concerns stainless steels having improved physical properties, most notably signicantly improved machinability. Specifically, it has been foundthat stainless steels containing from 11 to 30 weight percent chromium, balance substantially all iron, except for impurities and minor amounts of alloying elements, are vastly improved in machinability if amounts of manganese ranging from 0.80 to 4.0 weight percent are added to the steel in conjunction with sulfur in amounts ranging from 0.18 to about 0.50 weight percent. The amounts of manganese and sulfur added Within these stated ranges should be such that the manganese to sulfur ratio is between 3 to 1 and l0 to 1.

This is a continuation-in-part of our copending applications Ser. No. 472,756, filed on July 6, 1965 now abandoned, and Ser. No. 556,056, tiled on June 8, 1966 now abandoned, said former application being a continuationin-part of application Ser. No. 422,155, led Dec. 30, 1964 (now abandoned).

The AISI 400 series of stainless steels are straight chromium steels for general purpose applications requiring good corrosion resistance due, of course, to the relatively high chromium contents thereof. All of these straight chromium steels can be forged and machined, but some are relatively more forgeable or relatively more machinable than others. Machinability of stainless steels is an essential characteristic in a wide variety of end use applications, and certain of the stainless steels of the prior 'art have been devised with enhanced machinability particularly in mind. For example, AISI Type 416 stainless steel, containing, generally, 0.15% maximum carbon, 1.25% maximum manganese, 1.0% maximum silicon, about .06% phosphorus, 0.60% maximum molybdenum or zirconium, about l2 to 14% chromium, balance iron, contains a sulfur addition of about 0.15% minimum in order to take advantage of the well-known property of sulfur in machinability enhancement. Steels such as Type 416 are hardenable steels having a martensitic microstructure, and which contain optional additions of `molybdenum or zirconium, which are generally viewed as having a benecial effect, respectively, upon corrosion resistance and hot workability.

Cil

For many applications requiring extensive mechanical working, including machining, it is necessary that such stainless steels be amenable to annealing to a low hardness level of about 200 Brinell Hardness Number (BHN). Steels such as Type 416 are heat treatable to increased hardness levels; and this hardening characteristic is, of

course, quite important in many applications where the end use requires an alloy of relatively high hardness. Heat treated hardnesses of about 370 BHN are commonly attainable in such prior art steels.

As a consequence of the wide and increasing variety of required end use applications, the prior art has provided a host of variations in the basic stainless steel compositions, wherein the properties of the steels are varied, by compositional alterations and processing factors, to suit the specific steel for a particular desired use. Special emphasis has been placed by the prior art upon the enhancement of machinability of stainless steels, for example, by the addition of various quantities of sulfur and, in the case of some steels, by alteration of their microstructures, for example, by a particular balance of alloying elements and by special heat treatment. As a consequence, the steel-producing industry provides chromium stainless steels having fully martensitic structures, others with fully ferritic structures, as well as steels having a duplex structure, i.e., one containing a mixture of martensite and ferrite-the latter phase, being softer than ma'rtensite, adding substantially to the machinability of such steels as compared to steels of a martensitic nature. However, such microstructural alterations are accompanied by unavoidable property changes such as decreases in the maximum attainable hardness after heat treatment. Moreover, the addition of free-machining additives, such as sulfur, deleteriously affects other steel properties such as corrosion resistance, machined surface quality and forgeability.

Therefore, it is an object of the present invention to provide new chromium-containing stainless steels having enhanced machinability properties.

It is another object of the invention to provide new and improved methods for enhancing the machinability of stainless steels.

It is a particular object of the invention to provide new and improved straight chromium stainless steels having greatly enhanced machinability.

The foregoing and other objects of the invention will become more readily apparent by an inspection of the following detailed description and the accompanying drawings wherein:

FIGURE l is a graphical representation of the relationship between sulfur content and the machinability of a first series of chromium stainless steels;

FIGURE 2 is a graphical illustration of the effect of hardness of chromium stainless steels upon the machinability thereof, expressed as the dilference between observed test machinability and machinability calculated in accordance with a method herein disclosed;

FIGURE 3 graphically depicts the relationship between sulfur content of chromium stainless `steels and the difference between test and calculated machinability thereof FIGURE 4 is a graphical representation relating the dilference between test and calculated machinability to the combined effect of manganese, chromium, molybdenum and sulfur contents of chromium stainless steels;

FIGURE 5 is a graphical representation of the effect of molybdenum content of chromium stainless steels upon the machinability thereof, expressed as the difference between test and calculated machinability values;

FIGURE 6 graphically relates the relationship between machinability, expressed as the difference between test and calculated machinability, of chromium-containing stainless steels and the chromium equivalent (as hereinafter defined) thereof;

FIGURE 7 graphically illustrates the relationship between chromium equivalent and ferrite content in tested chromium stainless steels;

FIGURE 8 is illustrative of the relationship between ferrite content and drill machinability of tested chromium stainless steels;

FIGURE 9 is a graphical representation of the relationand the composition of sulfide inclusions therein, in terms of iron, chromium and manganese contents thereof;

FIGURE 19 graphically represents the relationship between the manganese-sulfur ratios of tested chromium stainless steels, in wrought condition, and the percent metal in sulfide inclusions therein, expressed as the chromium, iron and manganese contents thereof;

FIGURES 20-22 graphically relate the manganese content of the second series of tested stainless steels with ultimate tensile strength, offset yield strength, hardness, elongation, reduction of area and impact strength-under different heat-treated conditions of the steels so tested;

FIGURE 23 is a photographic representation of the results of exposure of several steel test samples, of varying ship between the sulfur content and machinability, exmanganese contents, to a water vapor corrosion test; and pressed as difference between actual and calculated ma- FIGURE 24 is a graphical representation of the effect chinability, of a second series of test alloys of the straight of manganese content of tested steel compositions upon chromium variety; weight loss thereof under several severely corrosive en- FIGURE 10 is a graph relating manganese content and vironmental conditions. drill machinability rating for a part of the aforesaid sec- Broadly, the stainless steel of this invention has about, ond series of test compositions; in weight percent, 11 to 30 chromium, up to 1.2 carbon, FIGURE 11 graphically relates the manganese-sulfur up to 0.60 or less than 0.35 molybdenum plus zirconium, ratio to the difference between test and calculated machinup to 1.0 silicon, 0.18 to 0.50 or 0.40 sulfur, 0.80 to 4.0 ability of the said second series of test alloys; manganese, up to 3.0 nickel and the balance iron, except FIGURE 121s a graphical representation of the correla- 25 for incidental impurities; the manganese to sulfur ratio tion between calculated and test drill machinability ratof the steel is between about 3 to 1 and about 10 to 1. ing for the tested steel compositions; The upper carbon limit may be 0.20 or 0.15 percent when FIGURE 13 is a graph relating manganese content, chromium is within the range of about 11 to 15 percent. sulfur content and machinability of the second series of The preferred upper limit for manganese in promoting tested alloys. machinability is 2.5 percent. Manganese may be present FIGURE 14 is a bar graph illustrating the relationship within the ranges of 1.20 to 4.0 percent, .80 to 1.25 perbetween machinabilities of certain prior art straight cent, over 1.25 to 2.5 percent, and over 1.25 to 5.0 percent. chromium stainless steels and those of this invention; Chromium contents within the range of 15 to 20 may be FIGURES 15A-F are photomicrographic reproductions used for purposes of obtaining a ferritic stainless steel; illustrative of sulfide inclusions in cast and forged heats for purpose of steel having alow ferrite content, chromium of straight chromium stainless steels having various manwithin the range of 11 to 15 percent may be used. For ganese-sulfur ratios; optimum machinability sulfur may be present within the FIGURES 16A-F are photomicrographic reproductions range of 0.35 to 0.50 percent. illustrative of the effect of heating on sulfide inclusions in As indicated hereinabove, improvements have preingots of straight chromium stainless steel compositions viously been made by the prior art in the machinability of of both low and high manganese-sulfur ratios; chromium-containing stainless steels by a close control FIGURE 17 is a graphical representation of the relaof the amounts and relative proportions of the various tionship between the manganese-sulfur ratio of as-cast and alloying elements present in the steel as well as by variaheat-treated straight chromium stainless steel compositions tions in heat treatment thereof during processing. Accordand the hardness (expressed as the impression diagonal ingly, an analysis was undertaken of the effect upon malength) of sulfide inclusions therein; chinability of the several alloying constituents of a FIGURE 18 is a graphical representation of the relagenerally 416 type stainless steel. For this purpose, a first tionship between the manganese-sulfur ratio of tested series of 48 heats were manufactured and evaluated, straight chromium stainless steels, in an as-cast condition, these heats having compositions as given in Table I.

TABLE I Composition Standard Hard- Test Calcu- Heat Sample Test Bar ness Machinlated Number Number Melt C Mn P S Si Ni Cr Mo Cul Ti Number BHN ability Machinability TABLE I-Continued Composition Standard Hard- Test Calcu- Heat Sample Test Bar ness Machlnlated Number Number Melt C Mn P S Si N1 Cr Mo Cu 1 Ti Number BHN ability Machiny ability 124942 22 0. 51 0. 017 0. 26 0. 27 0. 47 13. 03 0. 44 (O. 10) 214 221 92 92.0 23 0. 51 0. 017 0. 26 0. 27 0.47 13.03 0. 44 (0. 10) 214 221 93 92. 0 24 0. 48 0.016 0. 25 0.38 0. 28 12.81 0. 19 0. 10 214 225 97 97. 5 25 0. 50 0. 019 0. 29 0. 42 0.35 12. 99 0. 38 0. 12 214 234 96 97. 0 26 0. 47 0. 016 0. 26 0. 49 0. 24 12. 80 0. 18 0. 09 214 247 96 91. 5 27 0. 49 0. 016 0. 29 0. 62 0.36 12.90 0. 01 0. 0l 214 224 104 99. 5 28 0. 41 0. 017 0. 30 0. 43 0. 48 12. 93 0. 09 0. 15 214 202 100 94. 5 29 O. 49 0.018 0.26 0. 50 0.47 12.97 0. 10 0. 17 214 217 103 96. 5 30 0. 45 0. 015 0.34 0. 55 0. 42 12.84 0. 12 0. 11 214 235 99 99. 5 31 0` 47 0. 021 D. 30 0. 52 0. 50 13. 05 0. 34 0. 19 214 209 97 99. 5 32 0. 48 0. 017 0.34 0. 48 0.40 12.83 0. 14 0. 12 214 253 94 95.0 33 0.49 0. 015 0.32 0.35 0. 34 12.85 0.08 0. 16 214 216 100 107. 0 34 0. 58 0.018 0. 28 0. 56 0. 42 12.84 0. 15 0. 15 214 231 100 104. 0 35 0. 45 0. 017 0. 30 0. 31 0. 36 12. 83 0. 09 0. 08 214 239 100 96. 5 36 0.47 0. 018 0. 18 0.34 0.42 13. 21 0. 31 0. 11 214 229 91 89. 5 37 0. 50 0. 018 0. 31 0. 53 0. 35 12.86 0. 20 0. 12 214 227 103 101. 5 38 0. 46 0. 017 0.30 0.32 0.30 12. 82 0. 12 0. 13 214 225 102 99. 5 39 0. 60 0.021 0.32 0. 46 0. 42 13. 18 0. 44 0. 12 214 221 96 101. 0 40 0. 48 0. 017 0. 26 0.42 0.32 13. 12 0.34 0. 12 214 233 95 98. 5 41 0. 56 0.018 0.30 0. 42 0.22 13. 00 0. 18 0. 07 214 226 100 105. 5 42 0. 38 0. 014 0. 33 0. 29 0. 37 12. 48 0. 01 (0. 10) 214 192 99 92. 0 43 0.45 0. 013 0.33 0. 44 0. 11 12. 98 0.06 (0. 10) 214 197 101 106. 0 44 0.48 0.017 0. 19 O. 26 0. 33 12. 96 0. 21 0. 09 214 222 84 87. 0 45 0. 54 0. 012 0. 32 0. 30 0. 39 12. 49 0. 03 (0. 10) 0. 06 214 238 103 97. 0 46 0. 58 0.012 0.31 0.37 0. 38 12. 43 0.02 U. 10) 0. 04 214 216 102 103. 5 47 0. 58 O. 012 0. 31 0.37 0. 38 12. 43 0. 02 0. 10) 0.04 214 210 102 104. 0 55752 48 do 0. 109 0.47 0.016 0.31 0.95 0. 31 12. 87 0. 16 0.07 214 234 90 94. 0

1 Parentheses indicate that no analysis was made and that the amount was estimated.

The heats of Table I were prepared in the form of relatively large, commercial-size heats of abount 30,000 pounds per heat (denoted as large in Table I) or smaller, laboratory-size heats of about fifty pounds each (designated as small in Table I).

In the case of the larger heats, the steel was cast into 12-inch square ingot molds whereas the smaller heats were'cast into 12-pound ingots. The cast ingots were forged to -bar form, at least one bar from each heat was selected, the hardnessr thereof determined, as given in Table I, by standard measurement techniques, and each bar was then subjected to a drill machinability test, with results as set forth in Table I. In each case, the drill machinability value was determined by comparison with a standard AISI Type 303 stainless steel composition in bar form (Bar Nos. 214 and 364). Bar No. 364 had a rating of 99 when compared with Bar 214- to which a rating of 100 was assigned. Therefore, the test bars which were compared with Bar 364 as a standard were increased in rating by one point in order to make all test comparisons uniform. The drill test was made in a direction perpendicular to the longitudinal axis of the bar. A vertical drill press was utilized and operated at a uniform speed of 460 r.p.m. A 26-pound weight was suspended from a 7-inch lever arm to provide a constant load on a 1r-inch diameter drill. Twelve 0.400-inch holes were made with'three different drills to evaluate each test specimen. The typical drilling time for the standard bar was 14.5 seconds, and for most test bars the drilling time ranged between 11 and 18 seconds. The drill machinability was calculated by striking a ratio `between the standard bar drilling time and the test bar drilling time and multiplying by 100. Accordingly, test bars with good drill machinability showed a drilling time less than the standard and therefore have a drill rating greater than 100.

In order to evaluate the effect of chemical and hardness variations upon machinability, the drill machinability rating was considered as the dependent variable whereas chemistry and hardness were considered as independent variables, whereby correlation of drill machinability with steel composition may be described by an equation of the .form

y--k-t-axl-i-bxg (Equation 1) where y=drill machinability xhxz :independent variables The factors which were studied for their effect upon drill machinability included hardness, sulfur content, manganese content, manganese-sulfur ratio, molybdenum content, silicon content, chromium content, nickel content and chromium equivalent (as hereinafter described).

FIGURE 1, consisting of a graph relating weight percent sulfur content and observed values of test drill machinability, taken from Table I, is illustrative of the graphical approach taken in evaluating the effects of the above-mentioned factors. The wide scatter of the FIG- URE 1 data, showing a lack of good correlation of machinability with sulfur content indicates.` that at least one and possibly several other factors also affect machinability. Therefore, the analysis was extended yby assuming an effect for hardness, as well as for sulfur and reexamining the Table I data for possible improved correlation by adjusting the data to one hardness or to one sulfur level. Complete correlation was performed through a process of repeated analyses and adjustments. By such means, correlation was found between machinability and hardness, sulfur, manganese-sulfur ratio, molybdenum and chromium equivalent.

Graphs were constructed by calculating the machinability expected when all factors were considered except the one being evaluated, thereby to show the effect 0f such latter factor. The difference between such calculated machinability values `and the actually observed test machinability values was then plotted as a function of the factor `being examined. Illustratively, FIGURE 2 shows the effect, thus determined, of hardness on drill machinability. Considerable data scatter was observed which iS -believed due largely to variations of the heat size and ingot size, processing factors, as well as to some inevitable variations in the drill test itself. However, as shown in FIGURE 2, a linear relationship between hardness and machinability is clearly indicated.

FIGURE 3 similarly relates machinability and sulfur content, after adjustment of other factors affecting ma- 

